Radio receiver circuits



May 27, 1941. 1 R, W NT 2,243,141

RAIJIO RECEIVER CIRCUITS I Filed May 18, 1940 I 2 Sheets-Sheet 1 2'4057. 4554mm. I

I8 T044. F

NETWO-RK s/GTVAL SOURCE REGENERAT/VE AMPLIFIER LOCAL 086'.

INVEN TOR.

ROYA EAGANT,

ATTORNEY.

2. Sheets--Sheet 2 r0 REGEA/fkAT/VE AMPLIFIER T0 j NETWORK R. A. WEAGANTRADIO RECEIVER CIRCUITS Filed May 18,, 1940 LINEAR DETECTION PEG/0N FORDES/RED CARR/El? ONL Y- CAUSED FYI F. CARRIER/M1507 ION DETECTORCHAR/1C7 ERIS TIC REGION OF ACTION OF SELECTIVE IMPROVEMENT l/V osrscroeEFFICIENCY May 27, 1941.

l' J CONVERTER) INVENTOR. ROY WEAGANT ATTORNEY.

SIGNAL INPUT Patented May 27, 1941 FFHQE RADIO RECEIVER CIRCUITS Roy A.Weagant, Douglaston, Long Island, N. Y., assignor to Radio Corporationof America, a

corporation of Delaware Application May 18, 1940, Serial No. 335,892

8 Claims.

My present invention relates to modulated signal carrier receiversystems, and mor particularly to receivers utilizing selectiveimprovement in detector efficiency for desired signal carrier reception.

It is one of the main objects of my present invention to provide amodulated carrier detection system wherein selectivity is greatlyincreased by virtue of an electrical action termed selective improvementof detector efiiciency; the said action essentially comprising theinjection into the detector system of relatively slightly modulated, orunmodulated, energy of the desired carrier frequency, and the injectedcarrier energy being of such relatively great magnitude with respect tothe signal energy at the detector input network that detection is causedto be highly eiiicient for the modulated carrier energy of the desiredfrequency while detection is relatively ineii'lcient for signal energyof an undesired interfering frequency. Another important object of mypresent invention is to provide a radio receiver of the superheterodynetype which has high selectivity, high fidelity of reproduction and isrelatively free of adjacent channel interference; the receiver utilizinga minimum of stages prior to the audio network, and the first detectornetwork being preceded by solely a first detector stage, and the seconddetector network having means operatively associated therewith forselectively improving the detection efficiency solely for modulatedcarrier energy of the desired frequency.

Another important object of this invention is to provide in combinationwith a pair of diode 'rectifiers arranged for full Wave rectification,means for impressing upon the common input circuit of the diodescombined modulated carrier energies of desired and undesiredfrequencies, there being utilized a network for deriving from the signalenergies amplified oscillations of the desired carrier frequency, andthe amplified oscillations being applied to the diodes in such a mannerthat the rectification of the modulated carrier energy of desiredfrequency is relatively more efficient than the rectification of thesignal energy of undesired interfering frequency.

Still other objects of my present invention are to improve generally theefficiency, selectivity and fidelity of reproduction of radio broadcastreceivers, and more especially to provide a receiver of thesuperheterodyne type which not only possesses the above desirablecharacteristics, but is constructed in a compact manner and iseconomically manufactured and assembled.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings- Fig. 1 shows one embodiment of the invention,

Fig. 2 illustrates a modified arrangement,

Fig. 3 shows a modification of the second detector network, and

Fig. 4 graphically illustrates the selective improvement in detectionefiiciency which is utilized in the present invention.

Referring now to the accompanying drawings, wherein like referencecharacters in the different figures designate similar circuit elements,there is shown in Fig. 1 a receiving circuit of the superheterodyne typewhich may be constructed for use in the standard broadcast band of 550to 1600 kilocycles (kc). Of course, the invention may also be used inthe television and telegraph bands. When embodying the present inventionin a receiver, such a receiver need utilize but three tubes prior to theaudio network, and yet possess relatively greater selectivity and afidelity of reproduction of much higher order than usual broadcastreceivers. The numeral l designates a converter stage which may be ofthe combined local oscillator-first detector type. For example, thestage I can utilize, as is well known to those skilled in the art, apentagrid converter tube of the GA? type which produces in its outputcircuit 2 modulated carrier energy at intermediate frequency (IF). Suchnetworks are too well known to describe in any further detail, and it ismerely necessary to point out that the signal input grid of the GA? typetube will be connected to a tunable signal input circuit. The latterwill be coupled to any desired type of signal collector device,

such as a usual antenna circuit, a loop antenna,

a radio frequency distribution line, an automobile antenna device, orany other well known signal collection instrumentality. Of course, thetunable signal and local oscillation circuits are varied in unison bythe usual common tuning element, and by virtue of the electron couplingaction within the converter tube there may be developed across theoutput circuit 2 the I. F. voltage. Circuit 2 may be resonated to anydesired I. F. value in a range of 75 to 465 kc.

The tube following the converter stage is designated by numeral 3, andit may be one of the duo-diode-triode type. The two diode elements areutilized in the second detection network, while the triode section maybe utilized for audio amplification. However, if it is desired toutilize a double diode tube without the inclusion of any audio amplifierelectrodes, then a tube of the 61-16 type should be used. Consideringthe connections to the tube 3 in detail, the diode anodes 4 and 5 areconnected to opposite ends of the input circuit coil 6, while thecathode I is connected to ground through a bias resistor 8 shunted by anI. F, bypass condenser 9. The mid-point of coil 6 is connected to anydesired point on the bias resistor 8 through a path which includes theload resistor l U and the adjustable contact element l I, the I. F.by-pass condenser [2 being shunted across a major portion of theresistor l9.

In shunt with coil 6 are arranged condensers l3 and I4 connected inseries, and it will be understood that the circuit 6--l3l4 is resonatedto the operating I. F. value, and that primary circuit 2 is magneticallycoupled to the input coil 6 of the second detector network. I. F. signalenergy impressed on the detector input circuit is rectified in wellknown full wave rectification manner, and there is developed across theportion of the load resistor l shunted by condenser l2 both a directcurrent voltage component and an audio, or modulation voltagecornponent. An automatic volume control (AVC) circuit taps off thedirect current voltage component and numeral l denotes the AVGconnection. The latter includes the filter network l6 for eliminatingany pulsation voltage components from the AVG bias. The latter bias maybe applied to the signal grid of the converter tube so that there issecured the usual and well known automatic gain control action. Anotherreason for employing the AVC'circuit is to maintain the signal energyamplitude at the second detector of tube 3. A leak resistor I9 connectsgrid l8 to ground so that the proper negative bias may be applied togrid 18. The plate of tube 3 is connected to the positive terminal of adesired direct current source, and the audio transformer 21 has itsprimary winding included in the plate circuit of tube 3. The audiovoltage amplified by the amplifier section of tube 3 is amplified in oneor more audio amplifier stages, and may be finally reproduced in anywell known manner as by a loudspeaker.

According to the present invention, there is applied to the seconddetector network amplified oscillations of the I. F. value. Theamplified oscillations are derived from the I. F. circuit 2, and this isdone in the manner disclosed in my application Serial No. 234,938, filedOct. 14, 1938. The tube 22 has its input grid connected to the highpotential side of circuit 2 through an I. F. coupling condenser 23. Thecathode of tube 22 is connected to ground through a self-biasingresistor 24, the latter being shunted by an I. F. by-pass condenser. Theplate of tube 22 is connected to the positive terminal of a directcurrent source through the primary winding 25 of transformer 26. Thesecondary 21 of transformer 25 has one end thereof at ground potential,while its opposite end is connected by lead 28 to the junction ofcondensers l3 and I4. Each of windings 25 and 21 is shunted by its owncondenser, and each of the windings is fixedly tuned to the operating I.F. value. In this way the transformer 26 provides a pair of cascaded I.F.-tuned circuits, and the network is sharply tuned to the I. F. value.

The plate and grid of tube 22 are connected by the condenser 30, and thelatter is made adjustable so that the degree of regeneration provided bythe condenser 30 can be established at some predetermined magnitude. Itwill now be seen that the modulated I. F. carrier energy impressed uponthe grid of the regeneratively-coupled tube 22 has its modulation sidebands removed to a great extent from the I. F, carrier, and theresulting relatively slightly modulated I. F. carrier energy istransmitted through the highly selective transformer 26 and impressedupon the pair of diodes which comprise the second detector network. Theeffect of the regenerative tube is to remove the modulation to theextent of about 400 cycles. This causes reinforcement of the bass notes.Of course, substantially complete removal of modulation may be used.

The grid of tube 22 is automatically controlled in bias by connecting itto a desired point on load resistor in through a filter resistor 3| andlead 32. In the absence of signal energy the grid of tube 22 will beestablished at a normal negative bias with respect to the cathode. Uponthe impression of signal energy upon the full wave rectifier network,direct current voltage developed across the rectifier load resistor willbe applied over the AVG lead 32 to the grid of tube 22. In this way thegain of tube 22 is automatically controlled, and the magnitude of theinjected, or exalted, I. F. carrier oscillations is maintained at asubstantially constant magnitude with respect to the signal energyapplied to the input circuit of the second detector network. It will benoted that adjustment of tap I I along bias resistor 8 controls theinitial, or delayed, negative bias applied to the diode anodes 4 and 5.It will, also, be observed that the tube 22 not only functionssubstantially to strip the modulation off the I. F. carrier, but thatthe regeneratively-coupled tube functions as an amplifier of the I. F.carrier voltage. The I. F. carrier voltage oscillations injected intothe second detector network are in phase with the signal energy appliedto one of the anodes, while they are in opposite phase with the signalenergy applied to the other diode anode. This is the optimum operatingcondition to secure the selective improvement in detector efficiency.

The functioning of the second detector network will now be explainedwith particular reference to 'Fig. 4. In the latter there is shown afull line curve which graphically indicates the detector characteristicof a diode. It will be noted that Signal input is plotted against Audiooutput. As is well known, such a characteristic has a lower curvedportion for weak signal input which follows substantially a square law,while the upper portion thereof is substantially linear.

Detection over the square law portion of the curve is variable; it isextremely inefficient at the bottom, and equal to the linear portion atthe top. Extremely small amplification is used for the signal energiesapplied to the detector in order that even with interference present,for example more than 1000 times stronger than the signal, theinterference will be detected as nearly as possible at the bottom of thecharacteristic curve. When the exalted carrier energy is applied, andwhen the exalted carrier voltage is from ten to one hundred thousandtimes as strong as the signal energy, the latter will be detected on theefiicient part of the curve. Without injection of the exalted carrierenergy, detection occurs near the lower end of the characteristic. Sincethe slope of a square law curve at the exact zero point is zero, therelative efficiency at the top of the curve as compared to a very smallinput at the bottom approaches infinity. This means that an enormousincrease in desired signal output without effect on interference ispossible.

The shaded area of Fig. 4 is the Region of action of selectiveimprovement in detector efficiency. It is the curvature of this areathat makes the action possible. The circuits are soarranged that boththe signal and undesired interference are in the shaded region and asnear the bottom of the curve as is possible for ordinary detection. Theresults are obtainable with either a diode (having a curve as in Fig.4), or with a biased triode, which has only a square law of detection.The detector tubes, whether diodes or triodes, may be in balanced orunbalanced relation.

The injection of the amplified I. F. oscillations into the seconddetector network results in an automatic and enormous improvement of themodulated I. F. carrier energy of desired fre- 1 quency, while not inany Way affecting the inefficient detection of the undesired interferingcarrier energy. This high selective improvement in detection efiiciencyoccurs only when the ratio of the signal input to the second detector tothe magnitude of the injected amplified I. F. oscillations is very low.Experimental operation has completely verified the detection operationqualitatively depicted in Fig. 4. For example, at Miami, Florida, it waspossible to receive a station from Tampa (about 150 miles away) which ison 620 kc., and of 1 kw. power, even though the local station WIODoperated on 610 kc. (3 miles from receiver), with 1 kw. power. Ingeneral, any shape of detection curve will function provided that thereis a marked change in detection eificiency for strong signals ascompared to weak signals. The amplified I. F. oscillations injected intothe second detector network should, for a square law detector, be strongenough to carry the signal to the upper limit of the curved lowerportion of the characteristic, but not beyond it. While it is notbelieved necessary for the purposes of the present application tooutline any particular theoretical background for the selectiveimprovement in detection efficiency secured by my invention, it isstated, in addition to the aforementioned theoretical explanations, thatthe selectivity obtained is due to the action of the injected I. F.oscillations in greatly increasing the efficiency of signal detectionwithout at the same time increasing the efiiciency of the detection ofthe undesired interfering carrier energy.

Insofar as the AVG action of the receiver is concerned it will beobserved that the amplified I. F. oscillations injected into the seconddetector network will give rise to an augmented AVC bias, and,therefore, there will be an augmented AVC action in the receiver.Furthermore, by providing a delayed bias for the diode anodes 4 and 5,and the delay bias is produced by the voltage drop between ground andthe point on resistor 8 on which tap H is connected, the advantageresults that a threshold potential is set up for the desired andinterfering signal the strong exalted carrier energy, will be detectedon the linear-portion, or on the upper part of the curved portion. Itmay be noted in this connection that the accentuated I. F. carrier may,if desired, be of enormously greater value than the normal carriersimply by employing greater amplification in the circuit feeding theregeneratively-coupled tube. When the exalted I. F. carrier energy isinjected into the second detector network there is a great improvementin selectivity, and there is secured an equal improvement in fidelity ofreproduction due to both bass amplification and amplification of thetreble notes. a

It will now be seen that in addition to producing a very high order ofselectivity, the present arrangement results in an extremely high orderof sensitivity for a given number of tubes. Insofar as theregeneratively-coupled tube is concerned, it is pointed out that thespecific type of circuit shown is extremely stable, and capable of therequired I. F. oscillation-amplification.

In Fig. 2 there is shown a system of the type shown in Fig. 1, butdiffering therefrom in that there is provided additional I. F.amplification prior to the regenerativelycoupled tube 22'. Tube 22' isof the pentode type, and it is connected to function in the same mannershown in Fig. 1. However, the input electrodes of tube 22' are coupledto the tuned secondary circuit 40 of an I. F. transformer M. The tunedprimary circuit 42 of transformer M is resonated to the operating I. F.value, as is the secondary circuit 40. The anodes 4 and 5 of the fullwave rectifier are included in a tube 3', which includes a pair ofindependent triode sections. Thus, the cathode, grid 43 and plate 44provide one triode sec tion, while the cathode, grid 45 and plate 46provide the other triode section. Plates 44 and 46 are connected to theopposite ends of the tuned primary circuit 42, the mid-point of theprimary coil of transformer 4! being connected to the positive terminalof a direct current source.

The cathodes of tube 3' are connected to ground through the biasresistor 8 as in the case of Fig. 1. Grid 43 is connected to one side ofthe input coil 6 through an I. F. coupling condenser 50, the grid sideof the condenser 50 being connected to ground through a grid leakresistor 5!. Grid 45 is connected through I. F.

coupling condenser 52 to the low potential end of input coil 6, whilethe grid leak resistor 53 connects the grid side of condenser 52 toground. Thus grids 43 and 45 are established at a desired negative biaswith respect to the cathode of tube 3'. It will now be seen that thetriode sections of tube 3 provide a stage of push-pull I. F.amplification between the second detector input circuit and the inputelectrodes of the regeneratively-coupled tube 22. It is possible toproduce amplified I. F. oscillations whose amplitude relative to thesignal or interference input to the second detector network is of theorder of 100,000, assuming that the regenerative circuit has a Q of2000.

The discrimination against interfering signal voltage will be extremelyhigh, and the discrimination is still further increased by theutilization of the delay bias derived from resistor 8. The AVC bias isshown in Fig. 2 as applied to converter tube I, and to the signal gridof the regenerative tube 22'. AVG bias may, also, be applied to thesignal grids t3 and 45 of tube 3 by means of the direct current voltageconnection 50 and special filter resistors. The selective improvement indetection efliciency in this case is the same as described in connectionwith Fig. 1.

In Fig. 3 there is shown a modification of the second detection networkwherein the second detector is a double diode tube it of the 61-16 type.The diode anode H is connected to the high potential end of input coil6, while the diode anode i2 is connected to the opposite end of theinput coil. The detector load resistor is designated by the numeral 13,and is connected between the mid-point of coil 6 and ground. Theresistor 73 is shunted by the I. F. bypass condenser 14. The resistor 95is in series with resistor '13. AVG voltage and audio voltage are tappedoff from the load resistor 13 and used for the purpose shown in the caseof Fig. 1. The amplified I. F. oscillations from the regenerativeamplifier tube are impressed upon the double diode tube by coupling theprimary tuned output circuit of the regenerative tube, and which outputcircuit is denoted by numeral 80, to the tuned secondary circuit 8!. Thecircuits 80 and 8! are magnetically coupled, and are each resonated tothe operating I. F. value. The cathode lead of diode tube 10 isconnected to the midpoint of the coil of secondary circuit 8|, and thelead is established at ground potential;

Diode anode II is connected to one end of the tuned secondary circuit 8!by a very small condenser 90 adapted to pass the amplified I. F.oscillations, while the equally small I. F. coupling condenser 9!connects anode 12 to the opposite side of the circuit 6|. The advantageof the circuit arrangement shown in Fig. 3 resides in the fact thatexalted I. F. carrier energy is subjected to full wave rectification, asis the signal energy applied to the input circuit of the full waverectifier. Whereas in the arrangements of Figs. 1 and 2 the exalted I.F. carrier energy combines only with one half of the applied signalenergy,

in the arrangement of Fig. 3 the exalted carrier affects the signalenergy detection on both halves of the signal wave.

It is to be clearly understood that the present invention is equallyapplicable to the balanced type of detector shown in my aforesaid patentapplication. Further, the reduction of static by the present system issignificant.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In combination with a detection network provided with an inputcircuit tuned toa desired signal carrier frequency, a source of signalenergy coupled to the input circuit for impressing upon the inputcircuit signal energy of a relatively small magnitude, means operativelyassociated with said source for deriving therefrom signal carrier energyof said signal frequency, said last means being constructed to amplifythe derived energy to a relatively high amplitude with respect to theamplitude of the signal energy at said detection network, and means forapplying the amplified derived energy to said detector network wherebydetection of desired modulated carrier energy occurs with highefiiciency and detection of undesired interfering signal energy is ofrelatively low efiiciency.

2. In combination with a detection network provided with an inputcircuit tuned to a desired signal carrier frequency, a source of signalenergy coupled to the input circuit for impressing upon the inputcircuit signal energy of a relatively small magnitude, means operativelyassociated with said source for deriving therefrom signal carrier energyof said signal frequency, said last means being constructed to amplifythe derived energy to a relatively high amplitude with respect to theamplitude of the signal energy at said detector network, and means forapplying the amplified derived ener to said detector network wherebydetection of desired modulated carrier energy occurs with highefiiciency and detection of undesired interfering signal energy is ofrelatively low efiiciency andsaid deriving means comprising aregeneratively-coupled electron discharge tube having its inputelectrodes coupled to said source and its output electrodes coupled tosaid detection network.

3. In combination with a detection network provided with an inputcircuit tuned to a desired signal carrier frequency, a source of signalenergy coupled to the input circuit for impressing upon the inputcircuit signal energy of a relatively small magnitude, means operativelyassociated with said source for deriving therefrom signal carrier energyof said signal frequency, said last means being constructed to amplifythe derived energy to a relatively high amplitude with respect to theamplitude of the signal energy at said detector network, and means forapplying the amplified derived energy to said detector network wherebydetection of desired modulated carrier energy occurs with highefficiency and detection of undesired interfering signal energy is ofrelatively low efficiency, said detection network comprising a pair ofdiodes arranged as a full wave rectifier and having said input circuitas the input circuit of said rectifier.

4. In combination with a detection network provided with an inputcircuit tuned to a desired signal carrier frequency, a source of signalenergy coupled to the input circuit for impressing upon the inputcircuit signal energy of a relatively small magnitude, means operativelyassociated with said source for deriving therefrom signal carrier energyof said signal frequency, said last means being constructed to amplifythe derived energy to a relatively high amplitude with respect to theamplitude of the signal energy at said detector network, and means forapplying the amplified derived energy to said detector network wherebydetection of desired modulated carrier energy occurs with highefficiency and detection of undesired interfering signal energy is ofrelatively low efficiency said detector comprising a pair of diodesconnected as a full wave rectifier, and means responsive to theunidirectional voltage component of rectified signal energy forcontrolling the eficiency of said deriving means.

5. In combination with a detection network provided with an inputcircuit tuned to a desired signal carrier frequency, a source of signalenergy coupled to the input circuit for impressing upon the inputcircuit signal energy of a relatively small magnitude, means operativelyassociated with said source for deriving therefrom signal carrier energyof said signal frequency, said last means being constructed to amplifythe derived energy to a relatively high amplitude with respect to theamplitude of the signal energy at said detector network, and means forapplying the amplified derived energy to said detector network wherebydetection of desired modulated carrier energy occurs with highefiiciency and detection of undesired interfering signal energy is ofrelatively low efiiciency, said deriving means including an amplifierstage coupled to said source, and means responsive to the unidirectionalvoltage component of detected signal energy for controlling theefficiency of said deriving means.

6. In a superheterodyne receiver, a signal converter having anintermediate frequency output circuit, a second detector having anintermediate frequency input circuit coupled to the latter outputcircuit, said detector network consisting of a pair of diodes connectedwith said input circuit to provide a full Wave rectification network,

a regeneratively-coupled tube having an input circuit coupled to a pointbetween the converter and said rectification network, said regenerativetube output electrodes being coupled to said intermediate frequencyinput circuit, a highly selective coupling network, tuned to saidintermediate frequency value, arranged to couple the regenerative tubeoutput electrodes to said second detector input circuit, andmeans'responsive to the direct current voltage componentof rectifiedintermediate frequency energy for controlling the gain of saidregenerative tube.

'7. In a superheterodyne receiver, a signal converter having anintermediate frequency output circuit, a second detector having anintermediate frequency input circuit coupled to the latter outputcircuit, said detector network consisting of a pair of diodes connectedwith said input circuit to provide a full wave rectification network, aregeneratively-coupled tube having an input circuit coupled to a pointbetween the converter and said rectification network, said regenerativetube output electrodes being coupled to said intermediate frequencyinput circuit and said regenerative tube and its associated circuitsbeing arranged to amplify derived intermediate frequency oscillations toa point such that the amplitude of the latter is sufliciently greaterthan the signal voltage amplitude at the detector input circuit therebyto cause detection of the desired signal energy to take place along thelinear portion of the detection characteristic and the gain between saidconverter and detector being suficiently low to permit detection ofundesired signal energy along the lower inefficient portion of thedetection characteristic.

8. In combination with a demodulation network having a signal inputcircuit, means for producing oscillations of a frequency equal to thefrequency of desired signal energy at the demodulator input circuit,said demodulator network having a detection characteristic which followsa law other than linear for weak signal amplitude at the demodulationinput circuit, and follows a ROY A. WEAGANT.

